Aortic surgery that is required to replace diseased portions of the thoracic and abdominal aorta appears to be increasing. In the past, resection of the thoracic aorta and replacement of the resected section with a graft was performed as an emergency operation to repair a ruptured vessel due to gun or knife wounds or auto accidents and the like. With the advent of the newer non-invasive examining equipment and techniques such as CT and MRI it appears that aneurysms of the thoracic aorta and abdomen occur fairly often with the patient not being aware of the problem other than a "back problem" or shooting pains in the legs. Such aneurysms may occur anywhere along the thoracic or abdominal aorta from the arch of the aorta to its bifurcation into the femoral arteries. Most commonly it appears that such aneurysms occur at the bifurcation point and extend upward into the abdominal aorta.
Most of the aneurysms are caused by arteriosclerotic action on the walls of the aorta resulting in loss of elasticity and weakening of the aortal walls resulting in ballooning or localized dilation of the aorta. When such dilation approaches or exceeds a diameter of 6 cm. rupture of the aorta is imminent.
In addition to aneurysms, occlusion or partial blockage of the aorta may occur. Such occlusion is usually caused by progressive degenerative changes in the wall of the blood vessel.
Physiologically both conditions result in loss of blood supply to the lower part of the body which if below the renal arteries can result in gangrene of the lower extremities. If above the renal arteries, loss of kidney function may occur because of the reduction of blood supply to the kidneys.
A common surgical repair technique that is currently used is to clamp the abdominal aorta above the aneurysm or occlusion and at the femoral arteries shutting down blood flow to the lower extremities and abdominal organs and lower spinal cord. If the aneurysm/occlusion is in the renal, lumbar and mesenteric arteries area of the abdomen, clamping of the aorta can be done in the exposed abdomen. However, if the lesion extends into the thoracic cavity one side of the thoracic cavity must be exposed and the thoracic aorta proximally clamped above the diaphragm. Even with modern surgical techniques the length of time such shut down extends is usually longer than one hour. During this time metabolism of the now stagnated blood continues resulting in ischemia and production of plasma purines, lactic acid and several other toxic metabolites which have deleterious vasoactivity. Further, the effect of clamping severely increases blood pressure above the clamp resulting in substantial strain on the heart.
When a graft has been inserted and blood vessels connected, the clamps are removed allowing instantaneous return of blood flow. This most often results in dangerous hypotension, blood acidosis and kidney vasoconstriction. Appropriate pharmacology, if rendered quickly, can manage the hypotension and acidosis but the restriction of blood flow through the kidneys and their hemodynamic phenomena can be serious and life threatening.
Through the years numerous external shunts of the prior art have been developed to act to transport arterial blood around the closed off section of the aorta to be connected to the organ and femoral arteries to prevent such problems. Such shunts require additional surgery for example to expose and insert a shunt into the subclavian arteries and subsequently into the femoral and organ arteries. Much data has been collected to identify the value of such shunts as has data refuting the same because of problems caused by additional surgery and time required therein.